1. Field of the Invention
The present invention relates to a holographic filter for protection from radiation, notably laser radiation. It can be applied especially to the making of glasses or visors to protect the eyes efficiently from a single-line or multiple-line laser illumination while at the same time providing for efficient transmission of the rest of the visible spectrum, so as to preserve the quality of comfortable viewing through the glasses or visors. More generally, it can be applied to the protection of any physical sensor that could be destroyed or damaged by radiation, for example laser-emitted radiation.
2. Description of the Prior Art
There are several existing solutions to the problem of attenuating radiation, for example by the absorption of one or more of their lines. The standard absorbent materials are not selective enough and any efficient system of protection causes considerable deterioration in the transmission of the rest of the visible spectrum. For example, to obtain protection from a given line and to obtain an attenuation that corresponds to a density of over 5, the entire visible spectrum becomes very greatly attenuated. The above-mentioned value of density, which expresses the attenuation of the line, implies that, in this case, only a part smaller than 10.sup.-5 of the energy of the line to be attenuated is transmitted. More generally, a density equal to n means that the energy transmitted through the absorbent material is equal to 10.sup.-n of the energy received by this absorbent material.
The only selective absorbent materials known are those that let through the infrared, i.e. they filter only lines with a wavelength of less than about 520 nm, but these absorbent materials greatly reduce viewing quality, with the scene that is viewed having a yellow or even red appearance.
Another approach consists in using filtering devices made on the basis of thin layers. These layers are constituted by a stack of metal or dielectric layers representing a variation in index designed to deflect and reflect the light. In devices such as these, the width of the reflected spectral band is directly related to the angular field to be covered by the protective system. For example, if protection is envisaged for a wide angular field, of about 45.degree. for example, against a laser line (532 nm for example), it is necessary, for a given angle of incidence, of 30.degree. for example, to reflect a wide spectral band ranging from 490 nm to 570 nm, which causes deterioration in viewing quality. While this approach appears to be sufficiently selective as regards the angle of incidence, it is not so with respect to the wavelength.
Known approaches make it possible, in theory, to filter light rays selectively. However, their practical embodiments do not always give the expected results and are therefore not efficient.
The aim of the invention is to overcome the above-mentioned drawbacks, notably by enabling the selective and efficient filtering of one or more light rays on a wide angular range, while at the same time providing for efficient transmission of the rest of the visible spectrum.